The present invention relates to a hydraulic swivel drive for a grab which has two tong-like movable grab arms, in particular a two-shell grab, having two swivel shafts which are pivotally supported in a shell carrier, are parallel to one another and whose shaft ends are connectable to the grab arms, and having a shaft drive piece which is displaceably supported parallel to the swivel shafts and is in screw engagement with each of the two swivel shafts such that the swivel shafts are rotated in opposite directions by displacement of the shaft drive piece.
Such a swivel drive for a two-shell grab is known from DE 201 07 206 U1 in which the respective swivel shafts are part of a hydraulic motor which is surrounded by the shell carrier made as a closed housing. Tubular pieces are respectively seated on the two swivel shafts which swivel the grab arms or grab shells open and closed, with said tubular pieces being guided longitudinally displaceably, but rotationally fixedly and being in screw engagement with the respective swivel shaft such that a longitudinal displacement of the tubular pieces results in a rotation of the swivel shafts. The tubular pieces are connected by a common center piece with which they together form a piston which is received in the interior of the housing which forms the cylinder for the piston. The housing bounds a respective pressure space at both sides of this piston which can be charged with hydraulic fluid. Depending on which of the two pressure spaces is charged with hydraulic fluid, the piston and the tubular pieces provided thereon moves in the one or the other direction so that the swivel shafts swivel the grab shells secured thereto either open or closed.
Although the already known swivel drive for grabs has large advantages, is in particular very compact and small in construction and is protected from the rough surrounding influences during construction site operation due to the encapsulated construction of the hydraulic motors, it is capable of improvement in a plurality of aspects. On the one hand, only a limited center distance of the swivel shafts can be realized with this already known swivel drive. If the swivel shafts should have a large spacing from one another predetermined for the corresponding type of grab shells, very large forces arise on the pressure loading of the shaft drive piece acting as the piston which can hardly be mastered, and which the housing can hardly withstand with a reasonable construction design. On the other hand, this already known swivel drive requires expensive and complex sealings of the swivel drive piece acting as a piston with respect, first, to the swivel shafts and, second, to the housing.